The marine conservation deposits of Monte San Giorgio (Switzerland, Italy): the prototype of Triassic black shale Lagerstätten

Marine conservation deposits (‘Konservat-Lagerstätten’) are characterized by their mode of fossil preservation, faunal composition and sedimentary facies. Here, we review these characteristics with respect to the famous conservation deposit of the Besano Formation (formerly Grenzbitumenzone; including the Anisian–Ladinian boundary), and the successively younger fossil-bearing units Cava inferiore, Cava superiore, Cassina beds and the Kalkschieferzone of Monte San Giorgio (Switzerland and Italy). We compare these units to a selection of important black shale-type Lagerstätten of the global Phanerozoic plus the Ediacaran in order to detect commonalities in their facies, genesis, and fossil content using principal component and hierarchical cluster analyses. Further, we put the Monte San Giorgio type Fossillagerstätten into the context of other comparable Triassic deposits worldwide based on their fossil content. The results of the principal component and cluster analyses allow a subdivision of the 45 analysed Lagerstätten into four groups, for which we suggest the use of the corresponding pioneering localities: Burgess type for the early Palaeozoic black shales, Monte San Giorgio type for the Triassic black shales, Holzmaden type for the pyrite-rich black shales and Solnhofen type for platy limestones.

These materials gain increasing attention because of their usefulness for comparisons with related taxa from other Triassic localities.Especially in China, several localities have produced articulated skeletons, mostly of vertebrates, that resemble the Monte San Giorgio specimens not only morphologically, but also with respect to preservation and facies of the sedimentary matrix (e.g., Hu et al., 2011;Jiang et al., 2005Jiang et al., , 2009Jiang et al., , 2020;;Li, 2006;Lu et al., 2018;Rieppel et al., 2000;Sun et al., 2016;Wang et al., 2008).This has revealed that there are numerous Triassic localities worldwide with more or less similar depositional conditions and thus fossil preservation as at Monte San Giorgio.Consequently, Monte San Giorgio has become a reference in research on this type of conservation deposits (cf.Arif et al., 2019;Benton et al., 2013Benton et al., , 2022;;Frey et al., 2019;Seilacher, 1970Seilacher, , 1990;;Seilacher et al., 1985).
Accordingly, we briefly outline the properties of the Besano Formation, Cava inferiore, Cava superiore, Cassina beds and the Kalkschieferzone of Monte San Giorgio.We compare these to those of other Triassic conservation deposits worldwide.For this purpose, we characterized the six Swiss Triassic conservation deposits and 39 other globally important Phanerozoic Lagerstätten including Ediacara using various sedimentological and palaeontological traits.These were then evaluated using multivariate analyses.The aims were (1) to identify similarities and differences of these Lagerstätten; (2) to identify the palaeoecological conditions in order (3) to better understand under which circumstances the various types of Lagerstätten have formed.

Methods
Here, we follow the approach of Frey et al. (2019) in using the questionnaire published by Seilacher et al. (1985) to compare marine conservation deposits starting with the Ediacaran through a variance-covariance principal component analysis.The variables included information (often absence/presence) on marine basin size, sedimentary facies, palaeolatitude (Fig. 6), thickness of the succession, duration of the time interval in which the exceptionally preserved fossils formed, sea level, sediment structures, pyrite in sediment, faunal composition, trace fossils, infauna, epibenthos, pelagic macrofossils, death marches, landing marks, soft parts, cuticles, roll marks, current alignment, aragonite, internal moulds (pyritic or other), concretions, deformation, carbonization, soft part preservation mode (phosphatization, pyritization, silicification or in clay minerals), and preservation by obrution, stagnation or microbial mats.For the analyses, we omitted 'life position' , because this was uniformly coded as 0. The original dataset of Frey et al. (2019) included only 21 Lagerstätten.
For this study, we added data of 34 localities to allow a global comparison of Triassic conservation deposits, which represents an incomplete but representative sample of known Lagerstätten (Tables 1, 2).Our list is not intended to be all inclusive for global Lagerstätten but was assembled with the intention of assessing the characteristics of the Triassic Monte San Giorgio in a global framework.For the Early Triassic, we included the Paris biota (Thaynes Fm) from Idaho, USA (Brayard et al., 2017;Doguzhaeva et al., 2018), the fauna from Kap Stosch, Greenland (Wordie Creek Fm;Brinkmann et al., 2010;Kear et al., 2016), the famous fish nodules from the Middle Sakamena Formation of Madagascar (Beltan, 1996;Brinkmann et al., 2010;Kogan & Romano, 2016), the ichthyosaur-and thalattosaur-bearing faunas of the Sulphur Mountain Formation from British Columbia  (Bassani, 1886).PIMUZ T 1839, Besano Formation, Monte San Giorgio (A).The skull of the specimen (C) is covered by a thin pyrite crust and in the pelvic region (B), some phosphatized soft-tissue remains are discernible.Mixosaurus belongs together with Neusticosaurus to the most common reptile fossils of the Besano Formation.This specimen was excavated in 1931 and prepared in 2019 by Christian Obrist The marine conservation deposits of Monte San Giorgio and Alberta, Canada (Bastiaans et al., 2023a(Bastiaans et al., , 2023b;;Neuman, 1992Neuman, , 2015) ) and of the Vikinghøgda Formation in Svalbard (Hurum et al., 2018).
As far as the Late Triassic is concerned, we now included the Xiaowa Formation of Guanling, China (Jiang et al., 2005) as well as three European Lagerstätten.Austria has important conservation deposits of Late Triassic age, which received more attention recently.These are the Lunz Formation with the Polzberg biota (Lukeneder & Lukeneder, 2021, 2022, 2023) and the Seefeld Member of Wiestal (Hornung et al., 2019).From Italy, we included the Calcare di Zorzino (Tintori, 1992), which is renowned for the oldest pterosaurs and its splendid fish fauna.Of course, there are numerous more Triassic localities worldwide that yielded excellent fossils; a more comprehensive list was assembled by Brinkmann et al. (2010), but even this list needs an update.
As in Frey et al. (2019), we included the data of these 45 Lagerstätten listed in Tables 1 and 2 in principal component analyses on the variance-covariance matrix in PAST  2010) and Benton et al. (2013).Note that most of the conservation deposits included here lie in tropical or moderate latitudes, while occurrences of disarticulated materials are also known from boreal to arctic regions (not differentiated here) Table 1 Coding for the comparison of conservation deposits based on characteristics supposed by Seilacher et al. (1985), using data and modifications by Frey et al. ( 2019   Coding: for all characteristics 0: absent and 1: present (Hammer et al., 2001).Eigenvalues are listed in Table 3.
We then assembled data of the faunal composition and rough abundance estimates of the Triassic Lagerstätten in Table 4 with the eigenvalues in Table 5.We included rough estimates of relative abundances of most organism groups with a focus on eukaryotes, i.e. invertebrates are also included (for cephalopods, see, e.g., Airaghi, 1911;Rieber, 1969Rieber, , 1970Rieber, , 1973Rieber, , 1974;;Pieroni, 2022;Pohle et al. in press).Finer estimates of abundances can currently not be made since quantitative data are not available at the same precision for all localities and strata included in this analysis.We then ran another principal component analysis in PAST to compare the relative faunal composition of the Triassic sites (eigenvalues in Table 5).Additionally, we carried out hierarchical cluster analyses (paired group, Ward's method) using the same datasets.The layout of all biplots was made with CorelDraw X8.

Characteristics of the Konservatlagerstätten of Monte San Giorgio
Like several other Triassic conservation deposits, today's Monte San Giorgio was located at the margin of the Tethys in a tropical latitude during the Middle Triassic (Fig. 6; see, e.g., Lu et al., 2018;Benton et al., 2013).Lithologically, the Besano Formation is dominated by light grey to dark grey dolomitized limestones and black shales (e.g., Arif et al., 2019;Bassani, 1886;Baumgartner et al., 2001;Bernasconi, 1994;Felber, 2006;Furrer, 1995Furrer, , 2003Furrer, , 2004;;Röhl et al., 2001;Stockar et al., 2012).Most of the strata that yielded articulated vertebrate skeletons (sometimes with embryos or soft-tissue remains: Figs.2B, 5) are thin-bedded and laminated black shales.In these respects, this Lagerstätte is quite similar to several of those from China (Guanling Fm of Luoping and Panxian, Falang Fm of Xingyi, Xiaowa Fm of Guanling; Benton et al., 2013).The fossiliferous Besano Formation is about 16 m thick and overlies the Salvatore Dolomite (Stockar, 2010).Above, the San Giorgio Dolomite and the Ladinian Meride Limestone follow; the latter contains further beds of Lagerstätten quality such as the Cava inferiore and superiore as well as the Cassina beds and the Kalkschieferzone (Stockar et al., 2010).Except for the somewhat more carbonatic Kalkschieferzone (hence the name), the other four units are dominated by finely laminated limestones and black shales (Stockar, 2010).Depending on the clay content, the fossils (including the bones) are flattened to varying degrees.This applies particularly to the articulated skeletons, although those preserved in carbonatic strata display three-dimensionally preserved bones, occasionally even undeformed.For a recent account of the fossil content with a focus on vertebrates, see Rieppel (2019).Interestingly, the Besano Formation of Monte San Giorgio plotted somewhat separately from the other black shale deposits compared to the analysis of Frey et al. (2019) and in a different region than the other fossiliferous units at Monte San Giorgio (Fig. 7).In one of the cluster analyses (Fig. 8B), the Besano Formation formed a cluster with the Seefeld Member of Wiestal and the Cava superiore, while the other three units of Monte San Giorgio fell in a different place.In the PCA, the data points of all the Chinese Lagerstätten plot in the vicinity of the Cava inferiore and superiore, the Cassina beds and the Kalkschieferzone; this pattern was also found in the cluster analyses of Fig. 8.The data points of the classic black shales such as Holzmaden (Posidonia shale, Toarcian, Jurassic; Röhl et al., 2002), Bundenbach (Hunsrück slate, Emsian, Devonian; Bartels et al., 1998;De Baets et al., 2013) or Christian Malford (Oxford Clay, Jurassic; Wilby et al., 2008) lie in their own cluster (grey in Figs. 7, 8).In the PCA, the data point of the Besano Formation plots at the margin of the field of the Monte San Giorgio type conservation deposits and more or less between the fields occupied by Solnhofen type and the Holzmaden type Lagerstätten.This was expected considering that in all these deposits, flattened but articulated vertebrate skeletons are common, phosphatized soft-tissues are preserved, and pyritization occurs, albeit to differing extends (very common in the Hunsrück slate and the Posidonia shale while relatively rare in the Besano Formation: Fig. 5).Importantly, the facies of Monte San Giorgio is intermediate given the fact that it contains both laminated limestones and black shales.Accordingly, the Kalkschieferzone and Cava inferiore plot closer to the Solnhofen type in Fig. 7B.Fossil preservation, faunal composition (lack or scarcity of benthos), sedimentary facies, and the palaeogeographical setting suggest at least episodically anoxic conditions (Röhl et al., 2001;Stockar et al., 2012).

Table 3 Eigenvalues and variance of the principal components used for the comparison of conservation deposits
For the matrix, see Tables 1 and 2 PC   For the literature sources, see text and Table 2. Coding: 0: absent or rare; 0.5: moderately common; 1: relatively abundant.For abbreviations see caption of Table 2 Fig. 7 Classification of marine conservation deposits using a variation-covariation principal components analysis and the characterization of the Besano Formation of Monte San Giorgio (red margin).Data are listed in Tables 1 and 2. Modified after Frey et al. (2019).Note that among Triassic deposits, Monte San Giorgio, the Paris biota (Thaynes Fm) and Polzberg (Lunz Fm) are the only ones plotting close to the classic Black Shale Lagerstätten in A. A PCA 1 and 2; B PCA 2 and 3 The marine conservation deposits of Monte San Giorgio Fig. 8 Classification of marine conservation deposits using cluster analyses.A paired groups.B Ward's method.Note how in both cases, the Solnhofen and Holzmaden type Lagerstätten cluster well

Triassic marine conservation deposits worldwide
As mentioned above, numerous Triassic localities with preservation modes similar to those known from the fossiliferous beds at Monte San Giorgio have been discovered in South China in the last decades (e.g., Benton et al., 2013;Li, 2006;Lu et al., 2018).Some are from the Early Triassic (Chaoxian, Nanzhang, Wuming, Yuan'an), several from the Middle Triassic (Dingxiao, Guiyang, Fuyuan, Luoping, Luxi, Panxian, Qingzhen, Renhuai, Xingyi) and at least three from the Late Triassic (Guanling, Nylamu, Tingri).Although fossil preservation surprisingly often resembles that of the Besano Formation of Monte San Giorgio (Xiaowa Fm of Guanling, Daye Fm of Guiyang, Guanling Fm of Panxian, Falang Fm of Xingyi), the overall taxonomic composition is similar   4 The marine conservation deposits of Monte San Giorgio but differs in key aspects (Table 4,Figs. 9,10).Also, the respective abundances of the various vertebrate groups are not identical (Benton et al., 2013).The upper part of the Besano Formation yielded many fishes, Neusticosaurus (a pachypleurosaurid eosauropterygian) and Mixosaurus (an ichthyosaur), while some of the Chinese localities like the Xiaowa Fm of Guanling (Benton et al., 2013;Liu et al., 2013;Rieppel, 2019;Wang et al., 2008) are, e.g., rich in thalattosaurs.Furthermore, the huge pseudoplanktonic crinoid colonies of Traumatocrinus present at Guanling (Hagdorn et al., 2007) are unknown from the Alpine occurrences but are comparable to the driftwood crinoid colonies of Moroccocrinus of Late Devonian age from Morocco (Frey et al., 2018(Frey et al., , 2019;;Klug et al., 2003) and Seirocrinus found in the Jurassic Posidonia shale (e.g., Hess, 1999).
Our principal components analysis shown in Fig. 9 allows a classification according to the abundance of (i) more pelagic groups such as ammonoids, fishes and ichthyosaurs; (ii) more neritic groups such as eosauropterygians or tanystrophaeids or (iii) generally invertebrates.Remarkably, the data points of Cava inferiore, Cava superiore, Cassina Beds and the Kalkschieferzone again plotted closer to each other than to the point of the Besano Formation.This pattern is also seen in the cluster analysis in Fig. 10.When regarding the PC1/PC2-plot, the Besano Formation-point is in the field of Lagerstätten with abundant invertebrates, while when regarding PC2 and PC3, it falls quite central in the neritic groups/ reptile-dominated field, possibly due to the abundance of eosauropterygians, thalattosaurs and placodonts.The other four units of Monte San Giorgio fall in their own field in the PC2/PC3-plot with other neritic animal-dominated Lagerstätten.

Discussion
In our principal component analysis presented in Fig. 7, it is remarkable how well especially the platy limestones (Solnhofen type) separate from all other marine Lagerstätten, at least in PC1 and PC2.The German localities plot even closer together within this field (lower right corner in Fig. 7A).They formed their own cluster in both cluster analyses (Fig. 8).The Palaeozoic localities of the Cambrian and Ordovician also fell in a quite well delineated field, which comprises all of the Burgess type both in the PCA and in one of the cluster analyses (Fig. 8A).Likewise, the classic black shale occurrences (Holzmaden type) plot close to each other in the PCA and in both cluster analyses (Fig. 8), while the Monte San Giorgio type organic-rich deposits are somewhat scattered in the PCA (Fig. 7), occupying a large but separate field, thus suggesting a somewhat variable palaeoenvironment.Remarkably, this includes occurrences of Early, Middle and Late Triassic age.It is worth mentioning that this pattern remained stable even after several changes in the matrix.Unsurprisingly, the localities of the Chinese Triassic included here lie quite close to each other in the PCA and the cluster analyses.It is also noteworthy that, in the PC2/PC3-plot (Fig. 7B), the field of the Triassic Monte San Giorgio type Lagerstätten overlaps the fields comprising the black shales deposits of the Holzmaden and the Burgess type as well as the platy limestones of the Solnhofen type.The data points of the only Proterozoic Lagerstätten Ediacara and the obrution deposit Gmünd (Psilonotenton) usually fall more or less separate from the other points both in the PCAs and the cluster analyses.
The peculiar grouping of Monte San Giorgio type Lagerstätten (Figs. 7A, 8A, 9B), is remarkable.We suggest that the highly unusual palaeoecological conditions needed to produce the sedimentary facies of these fossiliferous beds can be explained by the long-term effects of the Permian-Triassic boundary mass extinction (Benton, 2016;Burgess et al., 2014).This created a palaeoenvironment with low oxygen conditions, euxinia, and acidification (Galfetti et al., 2007; Goudemand et al., 2019;Payne et al., 2004Payne et al., , 2012;;Romano et al., 2013) and other special conditions, that were unusually widespread throughout much of the Triassic.In turn, this is linked with low diversity benthos in many basins including a slow reef recovery (Benton, 2016).
In our second analysis, we focused on the faunal composition of the Monte San Giorgio type Lagerstätten.The Paris (Idaho; Thaynes Fm) and Polzberg biota (Austria; Lunz Fm) yield abundant and diverse invertebrates and plot near each other in both the PCA (Fig. 9) and the cluster analyses (Fig. 10).The other localities are distributed over the PCA-biplots according to whether their fauna is rather dominated by pelagic or neritic animals.A similar result was found in the cluster analyses, although the grouping differs somewhat (Fig. 10).In the PC1-PC2 plot, the Besano Formation of Monte San Giorgio is nested between the invertebrate dominated localities, while in the PC2-PC3 plot, it falls in the field with abundant thalattosaurs, placodonts and other more neritic animals, similar to some Chinese localities.
To some degree, the groupings correspond to our expectations, i.e. we knew that the localities of Madagascar (Sakamena Fm) and Ducanfurgga (Prosanto Fm) are fishdominated while invertebrates are very common in the biotas of Polzberg (Lunz Fm) and Paris (Thaynes Fm).The question arises to what extent the results depend on the sampling effort and the quality of the documentation of discoveries.Particularly, the discovery of larger vertebrates may depend on larger excavations over longer time spans.Future excavations in other localities should optimally be bed-by-bed like many excavations at Monte San Giorgio and should document abundance data (specimen counts per bed per surface area or rock volume).Although the Besano Formation at Monte San Giorgio has become one Triassic marine conservation deposit out of many, it is the pioneer of this kind of deposit in the Triassic and will remain an important reference in the future.Accordingly, we consider it adequate to name this kind of Triassic conservation deposits 'Monte San Giorgio type Lagerstätten' .

Conclusions
The conservation deposits of Anisian and Ladinian age of Monte San Giorgio, comprising the Besano Formation, Cava inferiore, Cava superiore, Cassina Beds, and the Kalkschieferzone, represent some of the first black shale conservation deposits of Triassic age that were thoroughly studied.Now, after a century of excavations and more than a century of research, these deposits begin to enjoy global scientific recognition (e.g., Etter, 2002a;Rieppel, 2019), and continue to produce valuable new information about the palaeobiology and evolution of Triassic vertebrates today.
With this paper, we want to highlight the key role of the conservation deposits of Monte San Giorgio: comparable to the pioneer role of the Burgess Shale for the Cambrian Lagerstätten or Solnhofen for the Mesozoic platy limestones, we highlight the pioneer role of the Besano Formation in particular as the prototype for Triassic Lagerstätten.Our simple comparison of 45 Fossillagerstätten worldwide employing principal component and hierarchical cluster analyses of 32 traits based on the list of Seilacher et al. (1985) confirm that the Besano Formation of Monte San Giorgio Lagerstätte is remarkably similar to other Triassic black shale deposits including, e.g., those of the Swiss Ducanfurgga and the South China block.The Triassic black shale deposits demonstrably occupy their own field separate from the Burgess type black shales, Solnhofen type platy limestones or Holzmaden type black shales.Accordingly, we introduce the term Monte San Giorgio type black shales.
The separate position of the Monte San Giorgio type organic-rich sediments can be explained by the mix of laminated limestones and black shales and the scarcity of benthics, as well as the rise of several new groups such as important clades of marine reptiles.This is to some extent very likely an effect of the long-term ecological impact of the Permian-Triassic boundary mass extinction and the recovery of marine biotas.
Concerning the fossil content of the Monte San Giorgio type Lagerstätten, we found the three main groups 'pelagic dominated' , 'neritic dominated' and 'invertebrate dominated' .For some of the included Lagerstätten, the position in the PCA-plots and the cluster analyses was expected, for some others we suspect that over the years, more vertebrates and particularly reptiles may be discovered with longer or increased collecting/sampling activities.Accordingly, the respective position might change in the future.The marine conservation deposits of Monte San Giorgio analyses have been published previously and the sources are provided in Tables 2 and 4.

Fig. 5
Fig. 5 Exceptionally preserved ichthyosaur Mixosaurus cornalianus (Bassani, 1886).PIMUZ T 1839, Besano Formation, Monte San Giorgio (A).The skull of the specimen (C) is covered by a thin pyrite crust and in the pelvic region (B), some phosphatized soft-tissue remains are discernible.Mixosaurus belongs together with Neusticosaurus to the most common reptile fossils of the Besano Formation.This specimen was excavated in 1931 and prepared in 2019 by Christian Obrist

Fig. 6
Fig. 6 Palaeogeographic map (modified after Scotese, 1997) with data from Brinkmann et al. (2010) andBenton et al. (2013).Note that most of the conservation deposits included here lie in tropical or moderate latitudes, while occurrences of disarticulated materials are also known from boreal to arctic regions (not differentiated here)

Fig. 9
Fig.9Classification of Triassic marine conservation deposits based on abundances of fossil groups using a variation-covariation principal components analysis and the characterization of the fossiliferous units of Monte San Giorgio (red margin).Data are listed in Table4.Note the similarity in faunal composition of Monte San Giorgio and the Chinese Lagerstätten.The Paris biota (Thaynes Fm) and Polzberg (Lunz Fm) are special in being more invertebrate dominated.The fish localities overlap with those with abundant reptiles (e.g., Ducan).A PCA 1 and 2; B PCA 2 and 3. Reptiles in the graph stands for other reptile groups

Fig. 10
Fig. 10 Classification of Triassic marine conservation deposits based on abundances of fossil groups using a hierarchical cluster analysis and the characterization of the fossiliferous units of Monte San Giorgio (red margin).Data are listed in Table 4

Table 4
Standardized coding for the comparison of Triassic conservation deposits based on faunal composition

Table 5
Eigenvalues and variance of the principal components used for the comparison of Triassic conservation deposits based on faunal composition